Effects of Manure Land Application Technologies and Timing on Environmental Fate of Four Antibiotics Commonly Used in Dairy Production.

Occurrence of veterinary antibiotics (ABs) in agricultural runoff has led to research efforts to reduce their transport from manure-applied fields. The objectives of this study were to investigate (1) impact of manure application methods and timing on the environmental fate of four widely used ABs in dairy production, including pirlimycin (PLY), tylosin (TYL), chlortetracycline (CTC), and sulfamerazine (SMZ); and (2) the long-term transformation patterns of ABs in manure-applied soils. Dairy manure spiked with PLY, TYL, CTC, and SMZ was applied at an agronomic N rate via traditional surface application and subsurface injection, a method designed for nutrient loss reduction. At days 0, 3, and 7 after manure application, a simulated rainfall (70 mm h^-1) was conducted to collect 30 min runoff. Soil samples were collected from 0-5 and 5-20 cm depth right after manure application and 0, 5, 14, 60, and 180 days after rainfall. Target ABs in soil and runoff water and sediment were quantified using UPLC/MS/MS. Results showed that < 3% of all four ABs left the field with surface runoff. Subsurface injection significantly reduced the mass loss of ABs in runoff water by 2-35 times compared to surface application (p<0.05). Regardless of manure application method, application at least 3 days before a rainfall event decreased the mass of ABs in the runoff water at least 6, 9, 10, and 28 times for TYL, PLY, CTC, and SMR, respectively. Subsurface injection concentrated ABs in the injection slits with limited horizontal diffusion within 15 cm, while surface application evenly distributed them throughout the soil surface, contributing to higher ABs loss through runoff. Long-term transformation of target ABs and their mass balance distribution in soil and runoff will be presented. This study provides evidence, for the first time, that manure application timing might significantly decrease surface runoff of ABs.